1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) technique for generating an image of an object by performing image reconstruction by detecting an X-ray transmitted through the object, and particularly, to an X-ray CT apparatus having a data acquisition system (DAS) and acquires information on an X-ray transmitted through the object required for image reconstruction as digital data.
2. Description of the Related Art
Recent X-ray CT apparatuses have an X-ray detector which is configured using an X-ray detection element array which is configured such that X-ray detection elements are two-dimensionally arranged. The X-ray detection element array has a tendency that the number of X-ray detection elements is increasing not only in a channel direction which is a rotational direction of the X-ray detector but also in a slice direction (line direction) which is crossed with the channel direction.
The X-ray CT apparatus is configured such that each X-ray detection element generates an X-ray detection signal (electrical signal), on which a DAS (see Japanese Patent Application Publication (Laid-open: KOKAI) No. 2006-15065 A) performs various kinds of signal processing such as a QV (Quantum of electric charge) conversion, an amplification, and an AD (Analog to Digital) conversion to generate projection data, which undergoes image reconstruction processing to produce a medical X-ray image, which appears on a display device.
FIG. 10 is a diagram illustrating a configuration example of a conventional X-ray CT apparatus.
FIG. 10 illustrates an X-ray detection element (PD) 9X and a DAS 6X of the conventional X-ray CT apparatus. FIG. 10 illustrates a configuration example in which 24 X-ray detection elements 9X on the X-ray detection element array share one QV chip (heavy lines in FIG. 10); and four X-ray detection elements 9X share one QV amplifier and one AD converter.
The DAS 6X has a QV amplifier unit 12X, an AD converter unit 13X, a first signal path 14X, and a second signal path 15X.
The QV amplifier unit 12X has QV chips as a plurality of IC chips and each QV chip has a plurality of QV amplifiers. FIG. 10 illustrates only a first QV chip having six QV amplifiers (QV 1 to QV 6) and a second QV chip having six QV amplifiers (QV 7 to QV 12). The X-ray detection element 9X detects a transmitted X-ray transmitted through the object and outputs an electrical signal reflecting an intensity of the transmitted X-ray. Each QV amplifier converts the electrical signal to a voltage signal and amplifies the voltage signal.
The AD converter unit 13X has a plurality of AD converters. FIG. 10 illustrates only twelve AD converters (ADC 1 to ADC 12). Each AD converter converts the voltage signal generated by a corresponding QV amplifier, to a digital signal.
The first signal path 14X forms a signal path starting at each X-ray detection element 9X and reaching the QV amplifier unit 12. The second signal path 15X forms a signal path connecting the QV amplifier unit 12X to the AD converter unit 13X. That is, the DAS 6X forms a signal path (starting at the first signal path 14X, passing through the QV amplifier unit 12X and the second signal path 15X, and reaching the AD converter unit 13X) for each X-ray detection element 9X, and acquires information on the transmitted X-ray as digital data.
Here, for the purpose of simplified structure, the DAS 6X is configured such that 24 (M=24) X-ray detection elements 9X continuing in a channel direction in the same line share one QV chip of the QV amplifier unit 12X. An X-ray detection element 9X in the m-th (m=1, 2, . . . , M) channel and the n-th (n=1, 2, . . . , N) line is expressed as an element [m, n]. For example, as illustrated in FIG. 10, the elements [1, 1] to [24, 1] share one QV chip.
Moreover, for the purpose of simplified structure, the DAS 6X is configured such that four X-ray detection elements 9X continuing in the channel direction and in the same line share one QV amplifier of the QV amplifier unit 12X. For example, as illustrated in FIG. 10, elements [1, 1] to [4, 1] share a QV 1 of the QV amplifier unit 12X.
Further, for the purpose of simplified structure, the DAS 6X is configured such that four X-ray detection elements 9X continuing in the channel direction and in the same line share one AD converter of the AD converter unit 13X. For example, as illustrated in FIG. 10, elements [1, 1] to [4, 1] share one ADC 1 of the AD converter unit 13X.
In general, the DAS 6X is configured such that signal processing characteristics differ for each circuit of the QV amplifier and the AD converter. Therefore, if a shared structure is used in which a QV amplifier is shared by the four X-ray detection elements 9X continuing in the channel direction and in the same line, the same QV amplifier has the same signal processing characteristics. Note that the DAS 6X is configured such that signal processing characteristics differ widely for each IC chip of a QV chip and the like. Therefore, if a shared structure is used in which a QV chip is shared by the 24 channels of X-ray detection elements 9X in the same line, signal processing characteristics differ widely between the two QV amplifiers (e.g., QV 1 and QV 7) each belonging to a different QV chip; while signal processing characteristics differ slightly (are similar) between the two QV amplifiers (e.g., QV 1 and QV 2) both belonging to the same QV chip.
FIGS. 11 and 12 are diagrams each illustrating a configuration example of a conventional X-ray CT apparatus.
FIGS. 11 and 12 each illustrate an X-ray detection element 9Y and a DAS 6Y of the conventional X-ray CT apparatus. FIGS. 11 and 12 each illustrate a structure example in which 16 X-ray detection elements 9Y on an X-ray detection element array share one QV chip (heavy lines in FIGS. 11 and 12); four X-ray detection elements 9Y share one AD chip (heavy lines in FIGS. 11 and 12); and one X-ray detection element 9Y corresponds to one QV amplifier and one AD converter.
The DAS 6Y has a QV amplifier unit 12Y, an AD converter unit 13Y, a first signal path 14Y, and a second signal path 15Y.
Here, the DAS 6Y is configured such that 16 (N=16) X-ray detection elements 9Y continuing in a line direction in the same channel share one QV chip of the QV amplifier unit 12Y. For example, as illustrated in FIG. 11, elements [1, 1] to [1, 16] share one QV chip.
In addition, the DAS 6Y is configured such that four X-ray detection elements 9Y continuing in the line direction in the same channel share one AD chip of the AD converter unit 13Y. For example, as illustrated in FIG. 11, elements [1, 1] to [1, 4] share one AD chip.
Moreover, the DAS 6Y is configured such that one X-ray detection element 9Y corresponds to only one QV amplifier. For example, as illustrated in FIG. 11, only element [1, 1] shares a QV 1 of the QV amplifier unit 12Y.
Further, the DAS 6Y is configured such that one X-ray detection element 9X corresponds to only one AD converter. For example, as illustrated in FIG. 11, only element [1, 1] corresponds to ADC 1 of the AD converter unit 13Y.
In general, the DAS 6X and 6Y illustrated in FIGS. 10 to 12 is configured such that signal processing characteristics differ widely for each IC chip of a QV chip and the like. Therefore, if a shared structure is used in which a QV chip is shared by the plurality of X-ray detection elements 9X, signal processing characteristics differ widely between the two QV amplifiers (e.g., QV 1 and QV 7 illustrated in FIG. 10) each belonging to a different QV chip; while signal processing characteristics are similar between the two QV amplifiers (e.g., QV 1 and QV 2 illustrated in FIG. 10) both belonging to the same QV chip.
More specifically, the DAS 6X illustrated in FIG. 10 is configured such that processing is performed on a group of four X-ray detection elements 9X continuing in the channel direction and in the same line on the X-ray detection element array under the same signal processing characteristics. In addition, the DAS 6X is configured such that a group adjacent to the above-mentioned group is signal-processed by the same QV chip, and thus processing is performed under similar signal processing characteristics. Moreover, the DAS 6X illustrated in FIG. 10 is configured such that a group of four X-ray detection elements 9X continuing in the channel direction and in the same line on the X-ray detection element array is signal-processed by the same QV chip and the same AD chip, and thus processing is performed under similar signal processing characteristics. Therefore, the DAS 6X and 6Y illustrated in FIGS. 10 to 12 cause an uneven distribution of signal processing characteristics on the X-ray detection element array, and thus artifacts are likely to appear noticeably.
Third-generation X-ray CT apparatuses are configured such that a channel of an X-ray detection element array is used in a fold back manner by sandwiching a channel at a center of field of view (FOV). At this time, in a channel region (particularly, 10-channel to 20-channel) near the center, the channel used in a fold back manner is restricted, and thus, artifacts due to an uneven distribution of signal processing characteristics of the DAS are likely to appear noticeably.
In recent years, image reconstruction performed by one scan using a widely used multi-line X-ray detection element array tends to produce such noticeable artifacts, and thus anti-artifact measures are required. As a method of removing such artifacts, a method can be considered of equalizing all signal processing characteristics of the QV chips, AD chips, QV amplifiers, AD converters, and the like constituting the DAS. However, the method is very difficult under the present technology and inevitably increases the cost of the X-ray CT apparatus and thus is unrealistic.